A mass mailing system generally comprises a mail inserting machine and a mail stacking machine. The mail inserting machine includes an envelope feeder and an enclosure document supply section. The envelope feeder is used to feed envelopes, one at a time, to an envelope insertion station. In the enclosure document supply section, a plurality of enclosure feeders is used to release enclosure documents to a chassis. The released documents are then gathered, collated and pushed by a plurality of pusher fingers to the envelope insertion station for insertion. Mail inserting machines are known in the art. For example, Roetter et al. (U.S. Pat. No. 4,169,341) discloses a mail inserting machine wherein documents are released onto a continuous conveyor mechanism to be collected and collated in a continuous process. If necessary, the collation can be folded such that each document is folded into two or more panels. Folding machines are known in the art. For example, Beck et al. (U.S. Pat. No. 4,701,233) discloses a method of folding a sheet by bulging a portion of the sheet and then folding the bulged portion through a roller nip. Marzullo (U.S. Pat. No. 4,875,965) discloses a folding apparatus wherein a buckle chute is used for stopping a sheet, causing the sheet to enter a roller nip for folding. After the enclosure documents are inserted into the envelopes, the filled envelopes are typically transported to another piece of equipment that seals the envelopes and affixes postage or prints a postage indicium on each envelope.
The filled envelopes are typically collected and loaded by an operator into mail trays or other forms of storage. This step in the mass mailing process has been found to be a “bottleneck”. One way to assist the operator in eliminating the bottleneck is to use an envelope stacking machine to automatically collect the filled envelopes into a stack so that the operator can remove the filled envelopes in stacks. One of the commonly used envelope stackers is an on-edge stacking apparatus. For example, Keane et al. (U.S. Pat. No. 6,388,204) discloses a mail stacking machine where a belt turn-up unit is used to turn the filled envelope from a horizontally facing direction to a vertical or “on-edge” position. The vertically oriented envelope is driven by a segmented roller into the back end of a vertical stack.
A typical mass mailing system is shown in FIG. 1. The mass mailing system 1 comprises a document collating station 10, a folding device 20, a mail inserting station 30, a mail input device 40 and a mail stacker 100. As shown in FIG. 2, a plurality of documents 12, which are released by a plurality of document feeders in the collation station 10, are collated into a stack or collation 14 as the documents are moved toward the mail inserting station 30. If necessary, the stack 14 is folded into a folded stack 22 prior to the documents being inserted into an envelope 32. After the stuffed envelope is scaled, it is referred to as a mailpiece 50. The mailpiece 50 is turned from a horizontally facing position to a vertical position by the mail input device 40. The vertically oriented mailpiece 50 is driven into the back end 64 of a vertical mail stack 60 in the mail stacker 100. The mailing system 1 also has a central control module 70, which is operatively connected to all of the sub-systems 10, 20, 30, 40 and 100 in order to coordinate the operation in the sub-systems. For example, if one of the sub-systems is non-functional, it may be necessary to stop all other sub-systems to avoid a jam. Furthermore, the central control module 70 is also used to control the document feeders in the collation station 10 so that only a predetermined number of documents are released to form a stack for mail insertion.
A typical stacking machine 100, as shown in FIG. 3, has an incoming mailpiece moving device 110 to move a mailpiece 50 released from the mailpiece input device 40 into a stacking deck 120 along a moving direction 250. The mailpiece 50 is further driven into the back end 64 of the mail stack 60. The stacking deck 120 has an upstream end 202 and a downstream end 204. As more mailpieces 50 are added to the back end of the mail stack 60, the stack 60 expands or grows toward the downstream end 204 of the stacking deck 120. As the stack expands, the pressure on the incoming mailpiece increases. In order to relieve the stack pressure, a conveyor belt 130 moving along a direction 230 is used to space out the stacked mailpieces, thereby making room for the next incoming mailpiece 50 to join the stack 60. At the same time, a paddle 140 is used to support the front end 62 of the stack, preventing the top mailpieces in the stack from falling toward the downstream end. The paddle 140 is movably disposed on a linear rod 144 for movement. The linear rod, which is substantially parallel to the moving direction 230, is fixedly mounted on rod mounts 146.
In order to achieve optimal operational efficiency, the rate at which the mailpiece input device feeding the mailpieces into the mail stacker is substantially equal to the rate at which the document stack is inserted into the envelope 32 in the mail inserting station. Furthermore, the insertion rate should also be equal to the rate at which the documents are released by each document feeder.
At the same time, the conveyor belt 130 is adapted to move at a certain speed in order to relieve the pressure in the mail stack. If the conveyor belt moves too slowly, the mailpieces will be packed too tightly. After a short time, a new mailpiece will not be able to squeeze into the stack and it will jam. If the conveyor belt moves too fast, the pack pressure will be too light and the mailpieces will move around as they settle down. In that situation, if an incoming mailpiece hits one of the mailpieces in the stack that is out of position, a jam may occur. Moreover, if the pack pressure is too light, the operator has to clear the stack more frequently. This makes the stacker less effective.
It is possible to set the speed of the conveyor belt 130 according to the feeding rate of the mailpiece input device 40. For example, the speed of the conveyor belt can be set in proportion to the feeding rate. Alternatively, the speed of the conveyor belt 130 can be manually adjusted by an operator so as to adjust the pack pressure. However, the pack pressure is not only affected by the feeding rate but by other factors as well. For example, when the mail stacker is first turned on, the motor that drives the conveyor belt is cold. The motor runs faster than when it has warmed up. This requires the operator to adjust the speed at the start up time and again after a few minutes as the machine starts to warm up. Manual adjustment, however, is not always consistent. It relies heavily on the experience of the operator.
One of the major pressure-related problems is caused by the variation of the thickness of the mailpieces. The stack pressure increases as the thickness of the mailpieces increases. If the speed of the conveyor belt is set for thin mailpieces, then this speed will not work well for thick mailpieces, because the stack pressure will rapidly increase, thus quickly causing a full jam. Even if a full jam does not occur, the late arriving mailpieces may not be stacked properly. As such, the edge of the mail stack will not form a straight line, causing a problem for the operator when the mail stack has one or more zip-code breaks. In a typical mail stacker, when the control system is designed to indicate to an operator a zip-code break, a mail stop is moved in front of a registration wall so that the incoming mailpieces stop at the mail stop instead of the registration wall. The thickness of the mail stop is usually about ¾″, and the edge of the newly-arriving mailpieces is supposed to shift about the same amount. The shift at the edge serves as an indication to the operator that this is an important point in the stack. When the stack pressure is too high, however, the mailpieces may not be properly stacked to indicate the zip-code break.
It is advantageous and desirable to provide a method and system to control the movement of the conveyor belt so that the pressure in a mail stack can be properly maintained.